Step-growth polymerization
- Step-growth polymerization
Step-growth polymerization is a polymerization process that involves a chemical reaction between multifunctional monomer molecules. In a step-growth reaction, the growing chains may react with each other to form even longer chains. This applies to chains of all lengths. Thus, a monomer or dimer may react in just the same way as a chain hundreds of monomer units long. This is in contrast to a chain-growth polymerization, where only monomers may react with growing chains (In chain-growth polymerization, two growing chains can't join together the way they can in a step-growth polymerization).
The most common class of step-growth polymerization is called condensation polymerization and the product a condensation polymer, because the chemical reaction by which the monomer molecules bond is often a condensation reaction that produces a small molecule byproduct. A multifunctional monomer is a molecule that has more than one potential reactive site by which it can form intermolecular chemical bonds. The easiest way to visualize a step growth polymerization is a group of people holding hands to form a human chain: each person has two hands (=reactive sites).
A pioneer in step-growth polymerisation is Wallace Carothers who invented nylon, a condensation product of hexamethylene diamine and adipic acid. Each monomer has two functional groups (two amino groups or two carboxyl group) and so each monomer can form an amide link with each of its neighbour.
The functionality of a monomer is the number of reactive sites. A functionality of 2 will yield a linear polymer. Examples:
* hexamethylene diamine and adipic acid create nylon
* terephthalic acid and ethylene glycol create PET In theory the polymerisation will continue to result in a single macromolecule. The relationship between the extent of the reaction and the average number of monomer units in a polymer chain is given by the Carothers equation. High molecular weight polymer is formed only at high degrees of conversion (extent of reaction). In practise the average length of the polymer chain is limited by such things as the purity of the reactants and the viscosity of the medium.
A monomer with functionality 3 will introduce branching in a polymer and will ultimately form a cross-linked macrostructure. The point at which this three-dimensional structure is formed is known as the gel point because it is signalled by an abrupt change in viscosity. One of the earliest so-called thermosets is known as bakelite.
It is not always water that is released in step-growth polymerization: in acyclic diene metathesis or ADMET dienes polymerize with loss of ethylene.
References
Wikimedia Foundation.
2010.
Look at other dictionaries:
Polymerization — An example of alkene polymerization, in which each Styrene monomer unit s double bond reforms as a single bond with another styrene monomer and forms polystyrene. In polymer chemistry, polymerization is a process of reacting monomer molecules… … Wikipedia
Chain growth polymerisation — Chain growth polymerization is a polymerization technique where unsaturated monomer molecules add on to a growing polymer chain one at a time [1]. It can be represented with the chemical equation: where n is the … Wikipedia
Addition polymerization — nM (monomer) ightarrow ( M ) n (polymer)where n is the degree of polymerization.CharacteristicsThe main characteristics are:* polymerisation process takes place in three distinct steps: # chain initiation, usually by means of an initiator which… … Wikipedia
polymerization — /peuh lim euhr euh zay sheuhn, pol euh meuhr /, n. Chem. 1. the act or process of forming a polymer or polymeric compound. 2. the combination of many like or unlike molecules to form a more complex product of higher molecular weight, with… … Universalium
industrial polymers, chemistry of — Introduction structure and composition of chemical compounds made up of long, chainlike molecules (molecule). What distinguishes polymers from other types of compounds is the extremely large size of the molecules. The size of a… … Universalium
industrial polymers, major — Introduction chemical compounds used in the manufacture of synthetic industrial materials. In the commercial production of plastics, elastomers, man made fibres, adhesives, and surface coatings, a tremendous variety of polymers are… … Universalium
Polymer — Appearance of real linear polymer chains as recorded using an atomic force microscope on surface under liquid medium. Chain contour length for this polymer is 204 nm; thickness is 0.4 nm.[1] A polymer is a large molecule (macromolecule … Wikipedia
surface coating — ▪ chemistry Introduction any mixture of film forming materials plus pigments, solvents, and other additives, which, when applied to a surface and cured or dried, yields a thin film that is functional and often decorative. Surface coatings… … Universalium
Molar mass distribution — In linear polymers the individual polymer chains rarely have exactly the same degree of polymerization and molar mass, and there is always a distribution around an average value. The molar mass distribution (or molecular weight distribution) in a … Wikipedia
Hydrolysis — is a chemical reaction during which one or more water molecules are split into hydrogen and hydroxide ions which may go on to participate in further reactions. [Compendium of Chemical Terminology, [http://goldbook.iupac.org/H02902.html… … Wikipedia
